Process for producing oxide thin films

ABSTRACT

This invention concerns a process for producing oxide thin film on a substrate by an ALD type process. According to the process, alternating vapour-phase pulses of at least one metal source material, and at least one oxygen source material are fed into a reaction space and contacted with the substrate. According to the invention, an yttrium source material and a zirconium source material are alternately used as the metal source material so as to form an yttrium-stabilised zirconium oxide (YSZ) thin film on a substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to oxide thin films produced by anALD method. In particular, the present invention relates toyttrium-stabilised zirconium oxide (YSZ) thin films.

[0003] 2. Description of Related Art The continuous decrease in the sizeof microelectronic components leads to the situation in which SiO₂ usedtoday as the gate oxide in metal oxide semiconductor field effecttransitions (MOSEFT) must be replaced with a higher permittivity oxide.This is due to the fact that in order to achieve the requiredcapacitances, the SiO₂ layer should be made so thin that the tunnelingcurrent would increase to a level affecting the functioning of thecomponent. This problem can be solved by using a dielectric materialhaving a higher dielectric constant than SiO₂. For example, thecapacitance of dynamic random access memory (DRAM) capacitors mustremain nearly constant while their size decreases rapidly, and thus itis necessary to replace the previously used SiO₂ and Si₃N₄ withmaterials which have higher permittivities than these and give highercapacitance density.

[0004] There is a number of materials exhibiting sufficiently highdielectric constant, but in addition to high permittivities, thesedielectric thin films are required to have, among other things, lowleakage current densities and high dielectric breakdown fields. Theachievement of both of these properties presupposes a dense and flawlessfilm structure. It is also important that the materials are stable incontact with silicon and can be exposed to the high post-treatmenttemperatures essentially without changes. Especially in the gate oxideapplication it is important that in the interface between silicon andthe metal oxide having high dielectric constant there are very fewelectrically active states. In the memory application it is importantthat the structure of the dielectric of the capacitor is stable, sincethe temperatures used for activation of implanted ions are high.

[0005] Zirconium oxide, ZrO₂ is an insulating material having a highmelting point and good chemical stability. ZrO₂ can be furtherstabilised by adding other oxides, the aim of adding other oxides is toeliminate the phase changes of ZrO₂. Normally, the monoclinic crystalform is stable up to 1100° C. and tetragonal up to 2285° C., above whichthe cubic form is stable. The stabilisation is typically carried out byadding yttrium oxide (Y₂O₃), but also MgO, CaO, CeO₂, In₂O₃, Gd₂O₃, andAl₂O₃ have been used. Previously, YSZ thin film layers have beenproduced, for example, by metal-organic chemical vapour deposition(MOCVD) (Garcia, G. et al., Preparation of YSZ layers by MOCVD:Influence of experimental parameters on the morphology of the film, J.Crystal Growth 156 (1995), 426) and e-beam evaporation techniques (cf.Matthee, Th. et al., Orientation relationships of epitaxial oxide bufferlayers on silicon (100) for high-temperature superconductingYBa₂Cu₃O_(7−x) films, Appl. Phys. Lett. 61 (1992), 1240).

[0006] Atomic layer deposition (ALD) can be used for producing binaryoxide thin films. ALD, which originally was known as atomic layerepitaxy (ALE) is a variant of traditional CVD. The method name wasrecently changed from ALE into ALD to avoid possible confusion whendiscussing about polycrystalline and amorphous thin films. Equipment forALD is supplied under the name ALCVD™ by ASM Microchemistry Oy, Espoo,Finland. The ALD method is based on sequential self-saturating surfacereactions. The method is described in detail in U.S. Pat. Nos. 4,058,430and 5,711,811. The growth benefits from the usage of inert carrier andpurging gases which makes the system faster.

[0007] When ALD type process is used for producing more complicatedcompounds, all components may not have, at the same reaction temperaturerange, an ALD process window, in which the growth is controlled. Mölsäet al. have discovered that an ALD type growth can be obtained whengrowing binary compounds even if a real ALD window has not been found,but the growth rate of the thin film depends on the temperature (Mölsä,H. et al., Adv. Mat. Opt. El. 4 (1994), 389). The use of such a sourcematerial and reaction temperature for the production of solid solutionsand doped thin films may be found difficult when a precise concentrationcontrol is desired. Also the scaling of the process becomes moredifficult, if small temperature changes have an effect on the growthprocess. Mölsä et al. (Mölsä, H. et al., Adv. Mat. Opt. El. 4 (1994),389) disclosed a process for growing Y₂O₃ by ALE-method. They usedY(thd)₃ (thd=2,2,6,6-tetramethyl-3,5-heptanedione) as the yttrium sourcematerial and ozone-oxygen mixture as the oxygen source material in atemperature range of 400-500° C. As already discussed, no ALE windowcould be found since the growth rate increased steadily from 0.3 Å/cycleto 1.8 Å/cycle with increasing temperature.

[0008] Ritala et al. (Ritala, M. and Leskelä, M., Appl. Surf Sci. 75(1994), 333) have disclosed a process for growing ZrO₂ by an ALD typeprocess. ZrCl₄ was used as the zirconium source material and water wasused as the oxygen source material. The temperature in the process was500° C. and the growth rate was 0.53 Å/cycle.

SUMMARY OF THE INVENTION

[0009] It is an object of the present invention to eliminate theproblems of prior art and to provide a novel process for producingyttrium-stabilised zirconium oxide (YSZ) thin films.

[0010] This and other objects together with the advantages thereof areachieved by the present invention as hereinafter described and claimed.

[0011] The present invention is based on the finding that yttrium oxideand zirconium oxide can be grown by an ALD type method so that the filmgrowth is in accordance with the principles of ALD so as to form anyttrium-stabilised zirconium oxide thin film. More specifically, theprocess for producing YSZ thin films is characterised by what is statedin the characterising part of claim 1.

[0012] A number of considerable advantages are achieved by means of thepresent invention.

[0013] The growth rate of the yttrium-stabilised zirconium oxide thinfilm is high, e.g., the growth rate of ALD thin film was approximately25% higher than would be expected based on the growth rates of ZrO₂ andY₂O₃.

[0014] The temperatures used in the present invention are low comparedwith the processes of prior art, which reduces the cost of theproduction process.

[0015] A film grown with the present process exhibits good thin filmproperties. Thus, the oxide films obtained have an excellentconformality even on uneven surfaces. The method also provides anexcellent and automatic self-control for the film growth.

[0016] The ALD grown yttrium-stabilised zirconium oxide thin films canbe used, for example, as insulators in electronics and optics. Forexample, in field emission displays (FED) it is preferred thatinsulating oxides which have a smooth surface, are used. It is alsopossible to use the YSZ thin films as solid electrolytes in gas sensorsand fuel cells. Particularly suitably the YSZ thin films are used asgate oxides in microelectronic devices, and as capacitor in dynamicrandom access memory (DRAM).

[0017] Next, the invention is described in detail with the aid of thefollowing detailed description and by reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 presents the growth rate of Y₂O₃ as a function of thegrowth temperature.

[0019]FIG. 2 presents the growth rate of Y₂O₃ as a function of the pulsetimes of the source materials.

[0020]FIG. 3 presents the thickness of a Y₂O₃ thin film in nm as afunction of the number of reaction cycles.

[0021]FIG. 4 presents the growth rate of ZrO₂ as a function of growthtemperature.

[0022]FIG. 5 presents the growth rate of ZrO₂ as a function of pulsetimes.

[0023]FIG. 6 presents the thickness of the ZrO₂ film as a function ofthe number of reaction cycles.

[0024]FIG. 7 presents the X-ray diffraction (XRD) patterns of ZrO₂ thinfilms grown at 300° C. and 450° C.

[0025]FIG. 8 presents the pulsing sequences of ZrO₂, YSZ and Y₂O₃ thinfilms.

[0026]FIG. 9 presents the growth rate of a YSZ thin film as a functionof Y₂O₃ content in the film.

[0027]FIG. 10 presents the XRD pattern of a YSZ thin film (thickness 90nm) grown on a (100) silicon substrate.

[0028]FIG. 11 presents the change of the d-value (interplanar spacing)of the (200) plane in the XRD pattern of a YSZ film as a function ofY₂O₃ concentration.

[0029]FIG. 12 presents the chloride concentration in a YSZ thin film asa function of the concentration of Y₂O₃.

[0030]FIG. 13 presents IR-spectra of (100) silicon substrate (a), YSZthin film (10 wt-% of Y₂O₃, thickness 120 μm, b) and a subtractedspectrum (c).

[0031]FIG. 14 presents the dependency of the wavenumber in themid-IR-area on the concentration of Y₂O₃.

[0032]FIG. 15 presents the Y/Zr ratio measured with different analysismethods.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Definitions

[0034] For the purposes of the present invention, an “ALD-type process”designates a process in which growth of material from gaseous orvaporized source chemicals onto a surface is based on sequential andalternating self-saturating surface reactions. The principles of ALD aredisclosed, e.g., in U.S. Pat. Nos. 4,058,430 and 5,711,811.

[0035] “Reaction space” is used to designate a reactor or reactionchamber in which the conditions can be adjusted so that growth by ALD ispossible.

[0036] “ALD window” is used to designate the temperature range in whichthe growth of a thin film takes place according to the principles ofALD. One indication of thin film growing according to the ALD principlesis the fact that the growth rate remains essentially constant over thetemperature range.

[0037] “Thin film” is used to designate a film which is grown fromelements or compounds that are transported as separate ions, atoms ormolecules via vacuum, gaseous phase or liquid phase from the source tothe substrate. The thickness of the film depends on the application andit varies in a wide range, e.g., from one molecular layer to 800 nm orup to 1 μm or even over that.

[0038] The Growth Process

[0039] According to the present invention, the oxide thin films areproduced by an ALD method. Thus, a substrate placed in a reactionchamber is subjected to sequential, alternately repeated surfacereactions of at least two vapor-phase reagents for the purpose ofgrowing a thin film thereon.

[0040] The conditions in the reaction space are adjusted so that nogas-phase reactions, i.e., reactions between gaseous reagents, occur,only surface reactions, i.e., reactions between species adsorbed on thesurface of the substrate and a gaseous reagent. Thus, the molecules ofoxygen source material react with the adsorbed metal source compoundlayer on the surface. This kind of growth is in accordance with theprinciples of ALD.

[0041] According to the present process the vapour-phase pulses of themetal source material and the oxygen source material are alternately andsequentially fed to the reaction space and contacted with the surface ofthe substrate fitted into the reaction space. The “surface” of thesubstrate comprises initially the surface of the actual substratematerial which optionally has been pre-treated in advance, e.g., bycontacting it with a chemical for modifying the surface propertiesthereof. During the growth process of the thin films, the previous metaloxide layer forms the surface for the following metal oxide layer. Thereagents are preferably fed into the reactor with the aid of an inertcarrier gas, such as nitrogen.

[0042] Preferably, and to make the process faster, the metal sourcematerial pulse and the oxygen source material pulse are separated fromeach other by an inert gas pulse, also referred to as gas purge in orderto purge the reaction space from the unreacted residues of the previouschemical and the reaction products. The inert gas purge typicallycomprises an inactive gas, such as nitrogen, or a noble gas, such asargon.

[0043] Thus, one pulsing sequence (also referred to as a “cycle” or“reaction cycle”) preferably consists essentially of

[0044] feeding a vapour-phase pulse of a metal source chemical with thehelp of an inert carrier gas into the reaction space;

[0045] purging the reaction space with an inert gas;

[0046] feeding a vapour-phase pulse of an oxygen source material intothe reaction space; and

[0047] purging the reaction space with an inert gas.

[0048] The purging time is selected to be long enough to prevent gasphase reactions and to prevent metal oxide thin film growth rates higherthan optimum ALD growth rate per cycle for said oxide.

[0049] The deposition can be carried out at normal pressure, but it ispreferred to operate the method at reduced pressure. The pressure in thereactor is typically 0.01-20 mbar, preferably 0.1-5 mbar.

[0050] The substrate temperature has to be low enough to keep the bondsbetween thin film atoms intact and to prevent thermal decomposition ofthe gaseous or vaporised reagents. On the other hand, the substratetemperature has to be high enough to keep the source materials in gasphase, i.e., condensation of the gaseous or vaporised reagents must beavoided. Further, the temperature must be sufficiently high to providethe activation energy for the surface reaction. When growing zirconiumoxide on a substrate, the temperature of the substrate is typically250-500° C., preferably 275-450° C., and in particular 275-325° C. Thetemperature range used for growing Y₂O₃ on a substrate is typically200-400° C., preferably 250-350° C. The YSZ films are typically grown at250-400° C., preferably at 275-350° C., and in particular at 275-325° C.

[0051] At these conditions, the amount of reagents bound to the surfacewill be determined by the surface. This phenomenon is called“self-saturation”.

[0052] Maximum coverage on the substrate surface is obtained when asingle layer of metal source chemical molecules is adsorbed. The pulsingsequence is repeated until an oxide film of predetermined thickness isgrown.

[0053] The source temperature is preferably set below the substratetemperature. This is based on the fact that if the partial pressure ofthe source chemical vapour exceeds the condensation limit at thesubstrate temperature, controlled layer-by-layer growth of the film islost.

[0054] The amount of time available for the self-saturating reactions islimited mostly by the economical factors such as required throughput ofthe product from the reactor. Very thin films are made by relatively fewpulsing cycles and in some cases this enables an increase of the sourcechemical pulse times and, thus, utilization of the source chemicals witha lower vapour pressure than normally.

[0055] The substrate can be of various types, for example sheet-formedor powder-like. Examples include silicon, silica, coated silicon, coppermetal, and various nitrides, such as metal nitrides.

[0056] The YSZ thin films grown according to the process of the presentinvention are typically (100) oriented.

[0057] Chlorine residues can be found in thin films comprising zirconiumand/or yttrium, when one or more of the source materials containschlorine. In the YSZ thin films produced according to the presentinvention the concentration of Cl in the films is typically 0.05-0.25wt-%. It was surprisingly found out in the connection of the presentinvention, that when the concentration of yttrium in the formed film waslow, i.e., below 20 wt-%, and in particular below 15 wt-%, the chloridecontent of the formed thin film was lower than that of a film consistingessentially of ZrO₂.

[0058] In the pulsing sequence described above, the metal sourcechemical is either a zirconium source material or an yttrium sourcematerial. Thus, in the growth process of the present invention, yttriumoxide and zirconium oxide are grown on a substrate.

[0059] According to a preferred embodiment of the present invention, anyttrium-stabilised zirconium oxide thin film is formed. Thus, during thegrowth of the thin film, at least one pulsing cycle described above willbe carried out using an yttrium source chemical as the metal sourcechemical, and at least one pulsing cycle described above will be carriedout using a zirconium source chemical as the metal source chemical.

[0060] The pulsing ratio between yttrium source chemical and zirconiumsource chemical can be selected so as to obtain the desired propertiesto the thin film. Typically, the pulsing ratio Y:Zr is from 1:10 to10:1, preferably from 1:5 to 5:1, more preferably from 1:3 to 3:1, andmost preferably the pulsing ratio is approximately 1:1.

[0061]FIG. 8 presents pulsing sequences which can be used for growingZrO₂, YSZ and Y₂O₃ thin films. In FIG. 8(b) the pulsing sequence forpulsing ratio Y:Zr=1:2 for the YSZ film is depicted.

[0062]FIG. 9 presents the growth rate of a YSZ thin film compared to avalue calculated for separate oxides as a function of weight percentageof Y₂O₃ in the film. The value to which the growth rate of YSZ iscompared is calculated by adding together the growth rates of Y₂O₃ andZrO₂ at each pulsing ratio of Y₂O₃:ZrO₂, and this calculated valuerepresents 100% in the figure. Thus, the figure shows which effect theY₂O₃:ZrO₂ pulsing ratio has on the growth rate and yttrium concentrationof the YSZ thin film.

[0063] According to one embodiment of the invention, when growing a YSZthin film the first pulsing cycle on a substrate is carried out using anyttrium source chemical as the metal source chemical.

[0064] According to another embodiment of the invention, when growing aYSZ thin film the first pulsing cycle on a substrate is carried outusing a zirconium source material as the metal source chemical.

[0065] The Source Materials

[0066] Gaseous or volatile compounds of yttrium and zirconium are usedas metal source materials in the process of the present invention.

[0067] Since the properties of each metal compound vary, the suitabilityof each metal compound for the use in the process of the presentinvention has to be considered. The properties of the compounds arefound, e.g., in N. N. Greenwood and A. Earnshaw, Chemistry of theElements, 2^(nd) edition, Pergamon Press, 1997.

[0068] The metal source material has to be chosen so that requirementsfor sufficient vapour pressure, the sufficient thermal stability atsubstrate temperature and sufficient reactivity of the compounds arefulfilled.

[0069] Sufficient vapour pressure means that there must be enough sourcechemical molecules in the gas phase near the substrate surface to enablefast enough self-saturating reactions at the surface.

[0070] In practice sufficient thermal stability means that the sourcechemical itself must not form growth-disturbing condensable phases onthe substrates or leave harmful levels of impurities on the substratesurface through thermal decomposition. Thus, one aim is to avoidnon-controlled condensation of molecules on substrates.

[0071] Further selecting criteria may include the availability of thechemical in a high purity, and the easiness of handling, inter al.,reasonable precautions.

[0072] In addition, the quality of the by-products resulting from theligand exchange reaction needs to be considered. It is important thatthe reaction product is essentially gaseous. By this it is meant thatthe by-products possibly formed in the ligand exchange reaction aregaseous enough to be moved from the reaction space with the aid of theinert purging gas, which means that they will not remain as impuritiesin the films.

[0073] 1. Yttrium Source Material

[0074] The yttrium source material is typically selected from the groupof materials having general formula (I) or (II):

YX₃  (I)

YX₃B  (II)

[0075] wherein

[0076] X is selected from the group of following:

[0077] i) diketone coordinated from oxygen (i.e., β-diketonate) offormula (III)

[0078] wherein

[0079] R′ and R″ are typically the same and are selected for examplefrom the group of linear or branched C₁-C₁₀ alkyl groups, in particularlinear or branched C₁-C₆ alkyl groups, and most preferably from thegroup of —CH₃, —C(CH₃)₃, —CF₃ and —C(CF₃)₃,

[0080] ii) cyclopentadienyl,

[0081] iii) derivative of cyclopentadienyl according to formula (IV):

C₅H_(5−y)R′″_(y)  (IV)

[0082] wherein

[0083] R′″ is selected for example from the group of linear or branchedC₁-C₁₀ alkyl groups, preferably C₁-C₆ alkyl groups, in particular methyl(—CH₃), ethyl, propyl, butyl, pentyl, and an alkyl having a longercarbon chain, alkoxy, aryl, amino, cyano and silyl group, and

[0084] y is an integer 1-5, and

[0085] B is a neutral adduct ligand, which binds to the center atom fromone or more atoms. Typically, B is hydrocarbon, oxygen-containinghydrocarbon (such as ether), nitrogen-containing hydrocarbon (such asbipyridine, phenantroline, amine or polyamine)

[0086] According to one embodiment of the present invention, Y(cot)Cp*(cot=cyclooctatetraenyl and Cp*=pentamethyl cyclopentadienyl) is used asthe yttrium source material.

[0087] According to a preferred embodiment of the present invention,Y(thd)₃ (thd=2,2,6,6-tetramethyl-3,5-heptanedione) is used as theyttrium source material.

[0088] 2. Zirconium Source Material

[0089] The zirconium source material is typically selected from thegroup of zirconium halides and zirconium compounds comprising at leastone carbon atom.

[0090] The zirconium source material is typically selected from thegroup having the general formula (V)

R₂ZrX₂  (V)

[0091] wherein

[0092] R is selected from the group of cyclopentadienyl (C₅H₅) and itsderivatives having the formula (IV).

[0093] The ligands R are optionally bridged (-Cp-A-Cp-), wherein A ismethyl, an alkyl group of formula (CH₂)_(n), n=2-6, preferably 2 or 3)or a substituted hydrocarbon such as C(CH₃)₂.

[0094] X is selected from the group of following ligands:

[0095] i) halides (F, Cl, Br, I),

[0096] ii) hydrogen (—H), linear or branched C₁-C₁₀ alkyl groups,preferably C₁-C₆ alkyl groups, in particular methyl (—CH₃), ethyl,propyl, butyl or a longer hydrocarbon chain,

[0097] iii) methoxy (—OCH₃) or other linear (e.g. —OC₃H₇) or branchedalkoxides,

[0098] iv) amines (—NR₂), and

[0099] v) acetates (—OCOR, e.g. —OCOCF₃).

[0100] According to one embodiment of the invention, X-ligands arecombinations of the compounds identified above. Thus, the zirconiumsource material is optionally Cp₂Zr(OR″″)_(x)Cl_(2−x) or Cp₂ZrClH).

[0101] The following preferred combinations of X and R can also be usedin the present invention:

[0102] X=R=Cl or Br, i.e, compound is a tetrahalide,

[0103] X=R=OR″, i.e., the compound is a zirconium alkoxide,

[0104] X=R=Cp, i.e., the compound is tetracyclopentadienezirconium,and/or

[0105] X=R=diketonate, coordinated from oxygen, having a formula (III).

[0106] Preferably, the zirconium source material used in the presentinvention is zirconium tetrachloride (ZrCl₄) or dicyclopentadienylzirconium dichloride (Cp₂ZrCl₂).

[0107] 3. Oxygen source material

[0108] The oxygen source material may be any oxygen compound usable inthe ALE technique. Preferable oxygen source materials include water,oxygen and hydrogen peroxide, and aqueous solutions of hydrogenperoxide. Ozone (O₃) is an especially preferable oxygen source material,also as mixture with oxygen (O₂). It is known on the basis of theliterature that, if ozone is used as the precursor for oxygen, a denserlayer of material is obtained from the forming oxides, and thereby thepermittivity of the oxide thin film can be improved.

[0109] One or more of the following compounds may also be used as theprecursor for oxygen:

[0110] oxides of nitrogen, such as N₂O, NO, and NO₂,

[0111] halide-oxygen compounds, for example chlorine dioxide (ClO₂) andperchloric acid (HClO₄),

[0112] peracids (—O—O—H), for example perbenzoic acid (C₆H₅COOOH) andperacetic acid (CH₃COOOH),

[0113] alkoxides,

[0114] alcohols, such as methanol (CH₃OH) and ethanol (CH₃CH₂OH), and

[0115] various radicals, for example oxygen radical (O⁻⁻) and hydroxylradical (⁻OH).

[0116] According to a preferred embodiment of the present invention, aYSZ thin film is grown by an ALD type method using Y(thd)₃ as theyttrium source material, dicyclopentadienyl zirconium dichloride(Cp₂ZrCl₂) as the zirconium source material and ozone or a mixture of O₃and O₂ as the oxygen source material.

[0117] According to another preferred embodiment, a YSZ thin film isgrown by an ALD type method using Y(thd)₃ as the yttrium source materialand a mixture of O₃ and O₂ as the oxygen source material, and zirconiumtetrachloride (ZrCl₄) as the zirconium source material and water as theoxygen source material.

[0118] The following examples illustrate the invention further.

EXAMPLES

[0119] Experimental Conditions and Analysis Equipment

[0120] In the examples, Y(thd)₃ and dicyclopentadienyl zirconiumdichloride (Cp₂ZrCl₂) (Strem Chemicals) were used as the metal sourcematerials. Y(thd)₃ was prepared according to the teaching of Eisentrautand Sievers (Eisentraut, K. J. and Sievers, R. E., J. Am. Chem. Soc. 87(1965), 5254). The source materials were analysed thermogravimetrically(TG/DTA, Seiko SSC 5200) at a pressure of 1 mbar.

[0121] The thin films were grown in MC-120 and F-120 reactors (ASMMicrochemistry Oy, Espoo, Finland) and N₂ (5.0, Aga) was used as thecarrier gas. Ozone, produced with an ozone generator (Fisher 502) fromO₂ (5.0, Aga), was used as the oxidiser. (100) oriented silicon (OkmeticOy, Finland) and lime glass were used as substrates. The growing ofseparate yttrium oxides and zirconium oxides was examined as thefunction of temperature and the suitability of the source materials wasconfirmed by experimenting with pulsing times in the range of 0.5-4seconds.

[0122] The crystallinity and orientation of the grown Y₂O₃, ZrO₂ and YSZthin films were analysed by X-ray diffraction (XRD, Philips MPD1880, CuK_(α)). The Y and Zr contents and the possible impurities weredetermined by X-ray fluoresence (XRF, Philips PW1480) using UniQuant 4.0software and by Scanning Electron Microscopy with Energy DispersiveX-ray analysis (SEM-EDX) using STRATA software. YSZ thin films were alsoanalysed by X-ray photon spectroscopy (XPS, AXIS 165, Kratos Analytical)using monochromated Al K_(α) radiation. Both wide scan spectra and HiRes(high resolution) spectra from areas C 1s, O 1s, Zr 3d and Y 3d weredetermined. The area of the measured sample was approximately 1 mm², andmeasurements were carried out from several points.

[0123] The thicknesses of the thin films were determined either withHitachi U-2000 UV-Vis spectrophotometer and with optical fitting methodas taught by Ylilammi, M. and Ranta-Aho, T. in Thin Solid Films 232(1993), 56 or by profilometry (the Y₂O₃ thin films) (Sloan DektakSL3030, Veeco Instruments) by etching with a solution of HCl theappropriate steps using a photoresist (AZ 1350H, Hoechst) as a mask.

[0124] The thin films were analysed also by Nicolet Magna-IR 750FT-IR-spectrophotometer using a DTGS detector and a DRIFTS accessory(Spectra Tech Inc.). From the samples prepared on a approximately0.5×0.5 cm² (100) silicon substrate mid-IR-area spectra were measuredwith 2 cm⁻¹ resolution and signal-averaging of 64 scans were used. Thebackground was measured with the diffuse alignment mirror of the device(SpectraTech no: 7004-015). The spectra of the silicon wafer with anative oxide was subtracted from the spectra of the samples. Theinterference in the spectra resulting from water and CO₂ residues in theIR apparatus was eliminated by purging with dry air. Smoothing of themeasured spectra was carried out when necessary.

Example 1

[0125] The Preparation and Analysis of Yttrium Oxide (Y₂O₃) Thin Films

[0126] Y₂O₃ thin films were grown by ALD method at a temperature of250-350° C. The growth rate of the Y₂O₃ thin films was 0.23 Å/cycle.

[0127] For Y₂O₃ thin film growth from Y(thd)₃ an ALD window was found inwhich the growth rate remained essentially constant in the temperaturerange of 250-350° C. The source material temperature was 120° C., thepulsing times were 0.8 s for Y(thd)₃ and 3.0 s for O₃, and the purgingafter each source material pulse lasted 1.0 s. This is also presented inFIG. 1, wherein the growth rate of Y₂O₃ in Å per cycle is depicted as afunction of growth temperature.

[0128]FIG. 2 depicts the growth rate of Y₂O₃ as A per cycle as afunction of the pulse times of the source materials. The figure showshow the growth rate remains essentially constant when the pulsing timeof Y(thd)₃ is approximately 0.5 s (during this experiment, the O₃ pulseas maintained at 1.5 s) or more and the pulsing time of O₃ isapproximately 1.0 s or more (during this experiment, the Y(thd)₃ pulsewas maintained at 0.8 s). The temperature of the yttrium source materialwas approximately 120° C., and the growth temperature was 300° C. Thepurging after each source material pulse varied from 0.8 to 2.0 s,increasing with increased pulse time.

[0129] In FIG. 3 the thickness of a Y₂O₃ thin film in nm is presented asa function of the number of reaction cycles. The film was deposited at300° C., and the temperature of the source material Y(thd)₃ was 120° C.The pulse times were 0.8 s for Y(thd)₃ and 3.0 s for O₃. The purgingafter each source material pulse lasted 1.0 s. It can be seen from FIG.3 that the thickness of the film is linearly dependent on the number ofgrowth cycles.

[0130] The Y₂O₃ films grown in the ALD window of 250-350° C. were (100)oriented. In the films grown at higher temperatures also (111) and (440)orientations were detected. The growth at temperatures higher than 400°C. yielded results similar to those obtained in prior art (Mölsä, H. etal., Adv. Mat. Opt. El. 4 (1994), 389). The orientation or crystallinityof the thin films did not vary according to the pulsing times of thesource materials.

Example 2

[0131] The Preparation and Analysis of Zirconium Oxide (ZrO₂) Thin Films

[0132] Zirconium oxide thin films were produced using Cp₂ZrCl₂ as thezirconium source material. The temperature of the source material was140° C. The ZrO₂ thin films could be grown according to the principlesof ALD at temperatures of 275-325° C. and at 400-450° C. In the firstrange, a growth rate of 0.48 Å/cycle was obtained, and in the secondrange, the growth rate was 0.53 Å/cycle.

[0133] This can also be seen in FIG. 4 which presents the growth rate ofa ZrO₂ thin film as a function of growth temperature. In thisexperiment, the temperature of the source material Cp₂ZrCl₂ was 140-150°C. The pulsing times of Cp₂ZrCl₂ and O₃ were 0.8 s and 3.0 s,respectively. The purging after each source material pulse lasted 1.0 s.

[0134] The pulsing times of the source materials were changed in someexperiments. A Cp₂ZrCl₂ pulse of 1.0 s saturated the surface of thesubstrate. An O₃ pulse of 1.5 s was needed to complete the reactioncycle. FIG. 5 depicts the growth rate of ZrO₂ in Å per cycle as afunction of the pulse time. The growth temperature was 300° C. and thetemperature of source material Cp₂ZrCl₂ was 140-150° C. The purging timewas 1.0 s. The figure shows how the growth rate remains essentiallyconstant when the pulsing time of Cp₂ZrCl₂ is approximately 0.7 s ormore (during these experiments, the pulsing time of O₃ was 3.0 s) andthe pulsing time of O₃ is approximately 1.5 s or more (during theseexperiments, the pulsing time of CpZrCl₂ was 0.8 s).

[0135] In FIG. 6 the thickness of a ZrO₂ thin film in nm is presented asa function of the number of reaction cycles. The film was deposited at300° C., the temperature of the source material Cp₂ZrCl₂ was 140-150° C.The pulse times were 0.8 s for Cp₂ZrCl₂ and 3.0 s for O₃. The purgingafter each source material pulse lasted 1.0 s. It can be seen from FIG.6 that the thickness of the film is linearly dependent on the number ofgrowth cycles.

[0136] XRF was used to analyse possible Cl residues present in the ZrO₂thin films. In the thin films grown on a silicon or glass substrate at250-275° C., approximately 0.1 wt-% of Cl was present. The thin filmsgrown at 300-325° C. exhibited a chlorine content of approximately0.06-0.07 wt-%. For films grown at temperatures higher than 325° C.chloride was not detected, i.e., the chlorine content was under thedetection limit, i.e., approximately 0.02 wt-% or less.

[0137] XRD was used to analyse the ZrO₂ films grown at differenttemperatures. The ZrO₂ thin films grown on silicon or glass substrate attemperatures below 300° C. were almost amorphous. Only very weak peaks,which could be interpreted as reflections of monoclinic ZrO₂ were shownin the film grown at 275° C. In the film grown at 300° C. the peakscould be identified as reflections of monoclinic or cubic ZrO₂ phase.When the growth temperature was up to 450° C., the monoclinic (−111)reflection was even stronger. The XRD patterns for the films grown at300° C. and 450° C. on a silicon substrate are presented in FIG. 7. Thepattern for the film grown at 300° C. is the one below. The thicknessesof films grown at 300° C. and 450° C. are 120 and 90 nm, respectively.The abbreviations used in the identification of the phases are asfollows: M=monoclinic, C=cubic. The identification was according toJCPDS cards 36-420 and 27-997 (Joint Committee on Powder DiffractionStandards (JCPDS), 1990).

Example 3

[0138] The Preparation and Analysis of Yttrium-Stabilised ZirconiumOxide Thin Films

[0139] YSZ thin films were grown at a temperature of 300° C. withdifferent pulsing programmes. In each pulsing programme the number ofpulsing sequences consisting of Y(thd)₃-pulse/purge/O₃-pulse/purge wasvaried with relation to the number of pulsing sequences consisting ofCp₂ZrCl₂-pulse/purge/O₃-pulse/purge.

[0140] The quality or the growth rate of the thin film did not depend onthe choice of the metal source material first deposited on the surfaceof the substrate.

[0141] The growth rate of the yttrium-stabilised zirconium oxide wasdependent on the Y/Zr pulsing ratio. If the growth rate of YSZ iscompared with the summed growth rates of separate oxides, it is noticedthat the at a pulsing ratio of 1:1, the growth rate is approximately 25%greater than the value calculated from the growth rates of separateoxides. When the number of yttrium pulsing sequences is increased, i.e.,when the yttrium content in the thin film increases, the growth rateapproaches the calculated value. This can also be concluded from FIG. 9.

[0142] The YSZ films grown at 300° C. were cubic, mainly (100) oriented,but, as FIG. 10 shows, also (111), (220) and (311) reflections weredetected. FIG. 10 discloses an XRD pattern for a YSZ thin film of athickness 90 nm. The film was grown on a (100) silicon substrate at 300°C. The Y/Zr pulsing ratio was 1:1. The phase is identified according toJCPDS-card 30-1468. The position of peaks in the XRD pattern change as afunction of the concentration of yttrium, since the size of the unitcell changes. The JCPDS reference value (card 30-1468) for the (200)reflection of Y_(0.15)Zr_(0.85)O_(1.93) is d=2.571 Å. FIG. 11 shows howthe (200) peak in the XRD pattern of a YSZ thin film changes as theY₂O₃/ZrO₂ ratio is changed. The dashed line in FIG. 11 is a referenceline drawn via the d-values of pure oxides obtained from literature.

[0143] The chlorine content of the YSZ films grown at 300° C. wasanalysed with XRF. At low concentrations of Y the amount of Cl in thefilms seemed to be slightly lower than in the films consistingessentially of ZrO₂. In the range 20-50 wt-% of Y₂O₃ in the thin filmthe increase in the amount of yttrium in the film resulted in anincrease of the amount of Cl incorporated in the film. This can also beseen in FIG. 12. The highest concentration of chlorine (0.23 wt-%) inthe YSZ thin film was detected when the yttrium oxide concentration was50 wt-%.

[0144] The IR-spectra measured for the YSZ films in the mid-IR-areamostly showed only the peaks resulting from the silicon substrate atdifferent wave numbers. The actual peaks resulting from the YSZ filmcould be detected by subtracting the IR-spectra of the Si-substrate (cf.FIG. 13). In the subtracting, the peak due to Si—O bond at 1100 cm⁻¹ didnot completely disappear. FIG. 14 shows how a distinct shift to higherwave numbers can be detected in the analysed films as the concentrationof yttrium decreases. The reference value of Y₂O₃ absorption is 613cm⁻¹.

[0145] A series of grown Y₂O₃, ZrO₂ and YSZ films was analysed withX-ray photoelectron spectroscopy (XPS). The contents of Y₂O₃ in thesamples were 0, 3, 10, 30 or 100 wt-%. Small amounts of carbon andoxygen (CO₂) were detected on the surface. This is typical for sampleshandled in air. The spectra measured from the surface was used tocalculate the atom compositions on the surface, and the atom ratio Y/Zrwhich was compared with the results obtained from X-ray fluorescence(XRF) measurements. This comparison is presented in FIG. 15, wherein theY₂O₃/ZrO₂ ratio calculated from XRF results is in the x-axis and theY/Zr ratio according to the XPS results is in the y-axis. The line isdrawn based on the XPS-HiRes measurements.

What is claimed is:
 1. A method for producing an integrated circuitcomprising: forming a zirconium stabilized yttrium oxide thin film overa substrate comprising a partially fabricated integrated circuit by anatomic layer deposition (ALD)-type process comprising alternatelyfeeding into said reaction space vapor phase pulses of a first yttriumsource material, a second zirconium source material, and at least oneoxygen source material capable of forming an oxide with the firstyttrium source material and the second zirconium source material.
 2. Themethod of claim 1, wherein the thin film is a capacitor dielectric. 3.The method of claim 1, wherein the thin film is a gate oxide layer.
 4. Amethod of forming a capacitor for a random access memory (RAM) device byan atomic layer deposition (ALD)-type process, said method comprising:providing a reaction chamber in an ALD reactor with a substratecomprising a partially fabricated capacitor; and forming a zirconiumstabilized yttrium oxide thin film over the substrate by a processcomprising: alternatively feeding into said reaction space vapor phasepulses of a first metal source material, a second metal source materialand at least one oxygen source material capable of forming an oxide withthe first metal source material and the second metal source material,wherein said first metal source material is a yttrium source materialand said second metal source material is a zirconium source material. 5.The method of claim 4, wherein the yttrium source material is Y(thd)₃,and the zirconium source material is dicyclopentadienyl zirconiumdichloride (Cp₂ZrCl₂), and the oxygen source material is selected fromthe group consisting of O₃ and a mixture of O₂ and O₃.
 6. The method ofclaim 4, wherein the random access memory device is a dynamic randomaccess memory (DRAM) device.
 7. A method of forming a gate oxide for atransistor comprising: providing a reaction chamber of an atomic layerdeposition (ALD) reactor with a substrate comprising a partiallyfabricated integrated circuit; and forming a zirconium stabilizedyttrium oxide thin film over the substrate by an ALD-type processcomprising: alternately feeding into the reaction space an yttriumsource material, a zirconium source material and an oxygen sourcematerial, wherein the oxygen source material is capable of forming anoxide with the yttrium source material and the zirconium sourcematerial.